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Sports Physics 6-8 / Week 09 / Wednesday
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Week 09 · Action and Reaction

Wednesday

Field Lab: Balloon rocket on a string
// Every push has a push back
⏱ about 20 min

Wednesday: Field Lab: Balloon Rocket on a String

Raven ties a long string from the fence post to the tree at the Field and pulls it tight.

Rocket threads a drinking straw onto the string, then tapes a blown-up balloon to the straw. He pinches the neck shut.

"Three, two, one," he counts, and lets go. The balloon zips along the string with a long hiss and stops near the tree.

"Again!" Rocket shouts. Raven raises the timer. "First, what do you notice? Which way did the air come out?"

"Backward, toward the fence. And the balloon went forward, toward the tree," Rocket says.

Nova hovers beside the still balloon. "That is a pair of forces," she says. "Now measure it instead of just watching."

Your mission

Today you build a balloon rocket and run it three times with a full balloon and three times with a half-full balloon.

You measure how far it travels and how long the run takes. Then you decide what the air did to the balloon.

  • A round balloon or two (a grown-up blows them up and watches them)
  • A drinking straw, clear tape and about 4 metres of string
  • Two anchors at the same height, such as two chair backs, a fence post and a tree, or two door handles
  • A tape measure, a timer, masking tape for marks and your Field Log
Safety first
Balloons are a choking risk for young children. A grown-up blows them up and keeps spare and popped balloons away from little ones.
Never let a balloon go near anyone's face. Stand beside the string, not at either end.
Tie the string at chest height or lower. Nobody climbs to tie it.
Stop if the balloon pops and a grown-up collects every piece.
  1. Thread the straw onto the string. Tie the string tightly between your two anchors so it is level.
  2. Blow up the balloon all the way and pinch the neck shut. Tape the balloon to the straw with two short strips.
  3. Slide the balloon to one end. Mark the start with masking tape. Ask your partner to hold the timer.
  4. Let go of the neck and start the timer. Stop timing when the balloon stops moving.
  5. Measure from the start mark to where the straw stopped. Record distance and time in the table.
  6. Repeat two more times with a full balloon. Then run three trials with a half-full balloon.
  7. Compute each speed: distance divided by time, in metres per second. One decimal place is enough.
PREDICT BEFORE YOU LAUNCH
  • Read the question.
  • Tap your answer.
The balloon pushes air out backward. Which way will the balloon move?
A half-full balloon pushes out less air. Compared with a full balloon, how far do you predict it will go?
TrialBalloonDistance (m)Time (s)Speed (m/s)
1full
2full
3full
4half-full
5half-full
6half-full

Reading the result

Your balloon is a tiny rocket. It pushes air out the back, and the air pushes the balloon forward. That forward push is thrust.

Nothing outside the balloon pushed it. The string only kept it straight. The force came from the balloon and air interacting.

If your balloon wobbled or stopped early, check the tape. A balloon neck pointing sideways sends thrust sideways.

Your speed numbers will vary from trial to trial. Finding the middle value of three trials is good science.

What the lab showsTrue or false?
The escaping air pushes forward on the balloon.?
The string pushes the balloon forward.?
The balloon and the air exert forces on each other.?
The balloon moved in the same direction as the escaping air.?
WHY THIS EXERCISEThe balloon rocket is the third law you can see: two objects, two forces, opposite directions.
What is the name for the forward push the air gives the balloon? Type one word.
WHY THIS EXERCISEThrust is a reaction force. The balloon earns it by pushing air backward.
Sketch your balloon on the string. Draw one arrow for the air leaving and one arrow for the balloon moving.

Great launch work. Tomorrow you will read NASA's own summary of all three laws of motion.

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